In the past Hybrid Integrated Circuits (HICs) suitable for high-voltage and high-performance applications were manufactured with Ti--Pd--Au (TPA) metallizations. However, the high cost of palladium and gold led to the development of lower cost substitutes for the noble metals in the TPA metallization system. The copper-based metallization, Ti--Cu--Ni--Au (TCNA), was developed as a low-cost alternative to the TPA metallization. The TCNA metallization eliminated the use of Pd and reduced the thickness of gold needed for a relatively low sheet resistance. Apart from the lower cost of the copper-based metallization, copper also offers the benefit of higher conductivity. See J. M. Morabito et al., "Material Characterization of Ti--Cu--Ni--Au (TCNA)--A Low-Cost Thin Film Conductor System", IEEE Transactions on Parts, Hybrids, and Packages, Vol. PHP-11, No. 4, December 1975, pp. 253-262. While Morabito taught complete elimination of the Pd layer, U.S. Pat. No. 4,016,050 suggests inclusion of Pd in the TCNA metallizations as an optional thin film between the Ti and Cu layers, and U.S. Pat. No. 4,109,297 teaches the use of a Pd thin film to improve adhesion between Ti and Cu layers forming Ti--Pd--Cu--Ni--Au (TPCNA) metallization in an attempt to overcome corrosion problems which were encountered in TCNA metallizations during the HIC fabrication. The corrosion manifested itself as a broad delamination of the copper layer from the titanium layer and as a combination of delamination and blistering between the copper and titanium layers. The delamination was observed after plating Cu--Ni--Au on the sputtered Ti--Cu film. In the combined form of corrosion (delamination and blistering), the delamination was accompanied by blistering of the TCNA film after a 30 minute heat treatment at 200.degree. C. In both types of corrosion, the addition of the sputtered Pd layer between the Ti and Cu layers eliminated the problem.
The use of Pd in the TPCNA metallization, however, led to yet another problem. Incomplete etching of the TPCNA metallization, as evidenced by metallic residues on the substrate, was frequently observed during the fabrication of the HICs. These residues occur mostly because the Pd layer is not etched chemically but rather is undercut and lifts off or flakes off (rejection etching) when the underlying Ti is chemically etched in a dilute HF solution.